Storage system with efficient detection and clean-up of stale data for sparsely-allocated storage in replication

ABSTRACT

An apparatus in one embodiment is configured to detect a failure in a data verification operation performed on at least a portion of at least one replicated storage volume. Responsive to the detected failure, the apparatus is further configured to perform bidirectional content-based signature comparisons between data pages of a source storage system and data pages of a target storage system. For one or more data pages of the source storage system for which respective content-based signatures are not found in the target storage system, the one or more data pages are sent from the source storage system to the target storage system. For one or more data pages of the target storage system for which respective content-based signatures are not found in the source storage system, the one or more data pages are designated as stale data and a clean-up operation is initiated for the stale data.

FIELD

The field relates generally to information processing systems, and more particularly to storage in information processing systems.

BACKGROUND

Many information processing systems are configured to replicate data from one storage system to another storage system, possibly at different physical sites. In some cases, such arrangements are utilized to support disaster recovery functionality within the information processing system. For example, an enterprise may replicate data from a production data center to a disaster recovery data center. In the event of a disaster at the production site, applications can be started at the disaster recovery site using the data that has been replicated to that site so that the enterprise can continue its business.

Data replication in these and other contexts can be implemented using asynchronous replication at certain times and synchronous replication at other times. For example, asynchronous replication may be configured to periodically transfer data in multiple cycles from a source site to a target site, while synchronous replication may be configured to mirror host writes from the source site to the target site as the writes are made at the source site. Storage systems participating in a replication process can therefore each be configured to support both asynchronous and synchronous replication modes.

Conventional approaches to data replication can be problematic under certain conditions. For example, it can be difficult to recover from a replication failure without requiring excessive amounts of data transfer between source and target storage systems. In addition, conventional replication failure recovery approaches do not effectively eliminate stale data that may arise from the failure. These and other issues can unduly increase the length of one or more subsequent replication cycles and thereby potentially prevent achievement of the desired recover point objective (RPO).

SUMMARY

Illustrative embodiments include storage systems with stale data detection and clean-up functionality that provides increased efficiency in recovery from replication failures. For example, some of these embodiments advantageously perform replication failure recovery in a manner that ensures that any stale data is effectively eliminated or otherwise cleaned up as part of the replication failure recovery, even in cases involving sparsely-allocated storage. Moreover, such stale data detection and clean-up functionality is implemented in some embodiments in a manner that conserves potentially scarce network bandwidth resources between source and target storage systems.

The source and target storage systems are illustratively implemented as respective content addressable storage systems, although other types of storage systems can be used in other embodiments.

In one embodiment, an apparatus comprises at least one processing device comprising a processor coupled to a memory. The processing device is configured to detect a failure in a data verification operation performed on at least a portion of at least one replicated storage volume. Responsive to the detected failure, the processing device is further configured to perform bidirectional content-based signature comparisons between data pages of a source storage system and data pages of a target storage system. For one or more data pages of the source storage system for which respective content-based signatures are not found in the target storage system, the one or more data pages are sent from the source storage system to the target storage system. For one or more data pages of the target storage system for which respective content-based signatures are not found in the source storage system, the one or more data pages are designated as stale data and a clean-up operation is initiated for the stale data.

In some embodiments, the source storage system comprises a first storage system associated with a first site selected as a winning site in a replication failure recovery process, and the target storage system comprises a second storage system associated with a second site selected as a losing site in the replication failure recovery process. The replication failure recovery process is illustratively initiated responsive to detection of the failure in the data verification operation, and a current replication cycle of an ongoing replication process carried out between the source and target storage systems is illustratively suspended in conjunction with initiation of the replication failure recovery process.

Performing bidirectional content-based signature comparisons between data pages of the source and target storage systems in some embodiments more particularly comprises sending hash handles and logical addresses for respective data pages of the source storage system to the target storage system, and determining if data pages having the hash handles exist at the corresponding logical addresses in the target storage system.

Any of the data pages for which it is determined that their respective hash handles do not exist at the corresponding logical addresses in the target storage system are sent from the source storage system to the target storage system.

Similarly, performing bidirectional content-based signature comparisons between data pages of the source and target storage systems in some embodiments further comprises sending hash handles and logical addresses for respective data pages of the target storage system to the source storage system, and determining if data pages having the hash handles exist at the corresponding logical addresses in the source storage system.

Any of the data pages for which it is determined that their respective hash handles do not exist at the corresponding logical addresses in the source storage system are designated as stale data in the target storage system.

The processing device in some embodiments is part of at least one of the source and target storage systems, and more particularly implements a storage controller of at least one of the source and target storage systems, although numerous alternative implementations are possible. For example, in other embodiments the processing device is implemented in a host device configured to communicate over a network with the source and target storage systems. Again, these are only examples, and alternative implementations are possible.

The storage volume illustratively comprises at least one logical storage volume comprising at least a portion of a physical storage space of one or more storage devices. The term “storage volume” as used herein is therefore intended to be broadly construed, so as to encompass a set of one or more logical storage volumes.

The storage volume in some embodiments is part of a consistency group subject to an ongoing asynchronous or synchronous replication process. Accordingly, the stale data detection and clean-up is illustratively performed in conjunction with replication of the consistency group from the source storage system to the target storage system.

The source storage system in some embodiments comprises a clustered implementation of a content addressable storage system having a distributed storage controller. The content addressable storage system in arrangements of this type is illustratively configured to utilize non-volatile memory storage devices, such as flash-based storage devices. For example, the storage devices of the source storage system in such embodiments can be configured to collectively provide an all-flash storage array. The target storage system can similarly comprise an all-flash storage array, or another type of content addressable storage system. Numerous other storage system arrangements are possible in other embodiments. Content addressable storage is therefore not required.

These and other illustrative embodiments include, without limitation, apparatus, systems, methods and processor-readable storage media.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an information processing system comprising source and target storage systems configured with stale data detection and clean-up functionality in asynchronous or synchronous replication in an illustrative embodiment.

FIG. 2 is a flow diagram of a process for stale data detection and clean-up in asynchronous or synchronous replication in an illustrative embodiment.

FIGS. 3A, 3B, 3C and 3D show examples of logical layer and physical layer mapping tables utilized in stale data detection and clean-up in asynchronous or synchronous replication in an illustrative embodiment.

FIG. 4 shows a content addressable storage system having a distributed storage controller configured with stale data detection and clean-up functionality in asynchronous or synchronous replication in an illustrative embodiment.

FIGS. 5 and 6 show examples of processing platforms that may be utilized to implement at least a portion of an information processing system in illustrative embodiments.

DETAILED DESCRIPTION

Illustrative embodiments will be described herein with reference to exemplary information processing systems and associated computers, servers, storage devices and other processing devices. It is to be appreciated, however, that these and other embodiments are not restricted to the particular illustrative system and device configurations shown. Accordingly, the term “information processing system” as used herein is intended to be broadly construed, so as to encompass, for example, processing systems comprising cloud computing and storage systems, as well as other types of processing systems comprising various combinations of physical and virtual processing resources. An information processing system may therefore comprise, for example, at least one data center or other cloud-based system that includes one or more clouds hosting multiple tenants that share cloud resources. Numerous different types of enterprise computing and storage systems are also encompassed by the term “information processing system” as that term is broadly used herein.

FIG. 1 shows an information processing system 100 configured in accordance with an illustrative embodiment. The information processing system 100 comprises a plurality of host devices 101, a source storage system 102S and a target storage system 102T, all of which are configured to communicate with one another over a network 104. The source and target storage systems 102 are more particularly configured in this embodiment to participate in an asynchronous or synchronous replication process in which one or more storage volumes are asynchronously or synchronously replicated from the source storage system 102S to the target storage system 102T, possibly with involvement of at least one of the host devices 101. The one or more storage volumes that are asynchronously or synchronously replicated from the source storage system 102S to the target storage system 102T are illustratively part of a designated consistency group.

Each of the storage systems 102 is illustratively associated with a corresponding set of one or more of the host devices 101. The host devices 101 illustratively comprise servers or other types of computers of an enterprise computer system, cloud-based computer system or other arrangement of multiple compute nodes associated with respective users.

The host devices 101 in some embodiments illustratively provide compute services such as execution of one or more applications on behalf of each of one or more users associated with respective ones of the host devices. Such applications illustratively generate input-output (TO) operations that are processed by a corresponding one of the storage systems 102. The term “input-output” as used herein refers to at least one of input and output. For example, IO operations may comprise write requests and/or read requests directed to stored data of a given one of the storage systems 102.

The storage systems 102 illustratively comprise respective processing devices of one or more processing platforms. For example, the storage systems 102 can each comprise one or more processing devices each having a processor and a memory, possibly implementing virtual machines and/or containers, although numerous other configurations are possible.

The storage systems 102 can additionally or alternatively be part of cloud infrastructure such as an Amazon Web Services (AWS) system. Other examples of cloud-based systems that can be used to provide at least portions of the storage systems 102 include Google Cloud Platform (GCP) and Microsoft Azure.

The storage systems 102 may be implemented on a common processing platform, or on separate processing platforms.

The host devices 101 are illustratively configured to write data to and read data from the storage systems 102 in accordance with applications executing on those host devices for system users.

The term “user” herein is intended to be broadly construed so as to encompass numerous arrangements of human, hardware, software or firmware entities, as well as combinations of such entities. Compute and/or storage services may be provided for users under a Platform-as-a-Service (PaaS) model, an Infrastructure-as-a-Service (IaaS) model and/or a Function-as-a-Service (FaaS) model, although it is to be appreciated that numerous other cloud infrastructure arrangements could be used. Also, illustrative embodiments can be implemented outside of the cloud infrastructure context, as in the case of a stand-alone computing and storage system implemented within a given enterprise.

The network 104 is assumed to comprise a portion of a global computer network such as the Internet, although other types of networks can be part of the network 104, including a wide area network (WAN), a local area network (LAN), a satellite network, a telephone or cable network, a cellular network, a wireless network such as a WiFi or WiMAX network, or various portions or combinations of these and other types of networks. The network 104 in some embodiments therefore comprises combinations of multiple different types of networks each comprising processing devices configured to communicate using Internet Protocol (IP) or other communication protocols.

As a more particular example, some embodiments may utilize one or more high-speed local networks in which associated processing devices communicate with one another utilizing Peripheral Component Interconnect express (PCIe) cards of those devices, and networking protocols such as InfiniBand, Gigabit Ethernet or Fibre Channel. Numerous alternative networking arrangements are possible in a given embodiment, as will be appreciated by those skilled in the art.

The source storage system 102S comprises a plurality of storage devices 106S and an associated storage controller 1085. The storage devices 106S store storage volumes 1105. The storage volumes 1105 illustratively comprise respective logical units (LUNs) or other types of logical storage volumes.

Similarly, the target storage system 102T comprises a plurality of storage devices 106T and an associated storage controller 108T. The storage devices 106T store storage volumes 110T, at least a portion of which represent respective LUNs or other types of logical storage volumes that are replicated from the source storage system 102S to the target storage system 102T in accordance with an asynchronous or synchronous replication process.

The storage devices 106 of the storage systems 102 illustratively comprise solid state drives (SSDs). Such SSDs are implemented using non-volatile memory (NVM) devices such as flash memory. Other types of NVM devices that can be used to implement at least a portion of the storage devices 106 include non-volatile random access memory (NVRAM), phase-change RAM (PC-RAM) and magnetic RAM (MRAM). These and various combinations of multiple different types of NVM devices may also be used. For example, hard disk drives (HDDs) can be used in combination with or in place of SSDs or other types of NVM devices.

However, it is to be appreciated that other types of storage devices can be used in other embodiments. For example, a given storage system as the term is broadly used herein can include a combination of different types of storage devices, as in the case of a multi-tier storage system comprising a flash-based fast tier and a disk-based capacity tier. In such an embodiment, each of the fast tier and the capacity tier of the multi-tier storage system comprises a plurality of storage devices with different types of storage devices being used in different ones of the storage tiers. For example, the fast tier may comprise flash drives while the capacity tier comprises hard disk drives. The particular storage devices used in a given storage tier may be varied in other embodiments, and multiple distinct storage device types may be used within a single storage tier. The term “storage device” as used herein is intended to be broadly construed, so as to encompass, for example, SSDs, HDDs, flash drives, hybrid drives or other types of storage devices.

In some embodiments, at least one of the storage systems 102 illustratively comprises a scale-out all-flash content addressable storage array such as an XtremIO™ storage array from Dell EMC of Hopkinton, Mass. Other types of storage arrays, including by way of example VNX® and Symmetrix VMAX® storage arrays also from Dell EMC, can be used to implement storage systems 102 in other embodiments.

The term “storage system” as used herein is therefore intended to be broadly construed, and should not be viewed as being limited to content addressable storage systems or flash-based storage systems. A given storage system as the term is broadly used herein can comprise, for example, network-attached storage (NAS), storage area networks (SANs), direct-attached storage (DAS) and distributed DAS, as well as combinations of these and other storage types, including software-defined storage.

Other particular types of storage products that can be used in implementing storage systems 102 in illustrative embodiments include all-flash and hybrid flash storage arrays such as Unity™ software-defined storage products such as ScaleIO™ and ViPR®, cloud storage products such as Elastic Cloud Storage (ECS), object-based storage products such as Atmos®, and scale-out NAS clusters comprising Isilon® platform nodes and associated accelerators, all from Dell EMC. Combinations of multiple ones of these and other storage products can also be used in implementing a given storage system in an illustrative embodiment.

In some embodiments, communications between the host devices 101 and the storage systems 102 comprise Small Computer System Interface (SCSI) commands. Other types of SCSI or non-SCSI commands may be used in other embodiments, including commands that are part of a standard command set, or custom commands such as a “vendor unique command” or VU command that is not part of a standard command set. The term “command” as used herein is therefore intended to be broadly construed, so as to encompass, for example, a composite command that comprises a combination of multiple individual commands. Numerous other commands can be used in other embodiments.

The storage controller 108S of source storage system 102S in the FIG. 1 embodiment includes replication control logic 112S and a signature generator 1145.

Similarly, the storage controller 108T of target storage system 102T includes replication control logic 112T and a signature generator 114T.

The instances of replication control logic 112S and 112T are collectively referred to herein as replication logic 112. Such replication control logic instances are also referred to herein as individually or collectively comprising at least a portion of a “replication engine” of the system 100.

The replication control logic 112 of the storage systems 102 controls performance of the asynchronous or synchronous replication process carried out between those storage systems, which as noted above in some embodiments further involves at least one of the host devices 101. The data replicated from the source storage system 102S to the target storage system 102T can include all of the data stored in the source storage system 102S, or only certain designated subsets of the data stored in the source storage system 102S, such as particular designated sets of LUNs or other logical storage volumes. Different replication processes of different types can be implemented for different parts of the stored data.

A given storage volume designated for replication from the source storage system 102S to the target storage system 102T illustratively comprises a set of one or more LUNs or other instances of the storage volumes 1105 of the source storage system 102S. Each such LUN or other storage volume illustratively comprises at least a portion of a physical storage space of one or more of the storage devices 106S. The corresponding replicated LUN or other storage volume of the storage volumes 110T of the target storage system 102T illustratively comprises at least a portion of a physical storage space of one or more of the storage devices 106T.

The replication control logic 112 of the storage systems 102 in some embodiments is configured to control the performance of a stale data detection and clean-up process in conjunction with ongoing asynchronous or synchronous replication, for example, in the manner illustrated in the flow diagram of FIG. 2. At least one of the host devices 101 in some embodiments can also include one or more instances of replication control logic and possibly also one or more signature generators.

The storage controllers 108 of the storage systems 102 should also be understood to include additional modules and other components typically found in conventional implementations of storage controllers and storage systems, although such additional modules and other components are omitted from the figure for clarity and simplicity of illustration.

It will be assumed for the following description of the FIG. 1 embodiment that there is an ongoing asynchronous or synchronous replication process being carried out between the source storage system 1025 and the target storage system 102T in the system 100, utilizing their respective instances of replication control logic 1125 and 112T.

An exemplary asynchronous replication process more particularly comprises a cycle-based asynchronous replication process in which a consistency group comprising one or more storage volumes is replicated from the source storage system 1025 to the target storage system 102T over a plurality of asynchronous replication cycles.

An exemplary synchronous replication process more particularly comprises a synchronous replication process in which host writes to a consistency group comprising one or more storage volumes are mirrored from the source storage system 102S to the target storage system 102T as the host writes are made at the source storage system 102S.

Other types of replication arrangements can be used in other embodiments. For example, the storage systems may be configurable to operate in both asynchronous and synchronous replication modes, with transitions between the modes controlled by their respective instances of replication control logic 1125 and 112T.

Other examples of replication processes that can be used in illustrative embodiments include active-active replication, in which one of the storage systems operates as a “leader” relative to another one of the storage systems operating as a “follower” in implementing consistent synchronous writes to both storage systems. Such active-active replication is considered a type of synchronous replication as that term is broadly used herein.

The system 100 is illustratively configured to provide what is referred to herein as “stale data detection and clean-up” in asynchronous or synchronous replication between the source storage system 102S and the target storage system 102T. These and other operations related to performance of stale data detection and clean-up functionality as disclosed herein are illustratively implemented at least in part by or otherwise under the control of the source and target instances of replication control logic 112S and 112T.

In accordance with the stale data detection and clean-up functionality, at least one of the storage systems 102 is configured to detect a failure in a data verification operation performed on at least a portion of at least one replicated storage volume. For example, in embodiments in which one or more storage volumes are being replicated from the source storage system 102S to the target storage system 102T as part of an ongoing asynchronous or synchronous replication process of the type described above, the target storage system 102T illustratively performs one or more data verification operations on at least a portion of at least one replicated storage volume. Other types and arrangements of data verification operations can be performed by other system entities in other embodiments. For example, in some embodiments, the source storage system 102S and/or one or more of the host devices 101 can perform or otherwise control or participate in a given data verification operation.

Responsive to the detected failure, the source and target storage systems 102 perform bidirectional content-based signature comparisons between data pages of the source storage system 102S and data pages of a target storage system 102T.

For one or more data pages of the source storage system 102S for which respective content-based signatures are not found in the target storage system 102T, the one or more data pages are sent from the source storage system 102S to the target storage system 102T.

For one or more data pages of the target storage system 102T for which respective content-based signatures are not found in the source storage system 102S, the one or more data pages are designated as stale data and a clean-up operation is initiated for the stale data. The clean-up operation can be initiated, for example, on a per-page basis, responsive to designation of each data page as stale data. As another example, groups of multiple data pages can be cleaned up once a threshold number of stale data pages is reached. Such clean-up operations can involve, for example, deleting, releasing or otherwise cleaning up the data pages that are designated as stale data. In some embodiments, initiating the clean-up operation illustratively involves the target storage system 102T issuing a trim operation and/or an unmap operation directed to a given data page or set of data pages designated as stale data. Other types of clean-up operations can be used in other embodiments.

The replicated storage volume illustratively comprises at least one logical storage volume that is part of a consistency group subject to the ongoing replication process carried out between the source and target storage systems 102.

The term “storage volume” as used herein is intended to be broadly construed, and should not be viewed as being limited to any particular format or configuration. The term “consistency group” as used herein is also intended to be broadly construed, and may comprise one or more other storage volumes.

The detection of the failure in the data verification operation illustratively comprises detection of a content-based signature mismatch between one or more data pages of the storage volume on the source storage system 102S and one or more corresponding data pages of the storage volume on the target storage system 102T.

The content-based signatures utilized in the detection of the failure in the data verification operation illustratively comprise respective hash digests of respective data pages of the replicated storage volume. A given one of the hash digests is generated in illustrative embodiments by applying a secure hashing algorithm to content of a corresponding one of the data pages of the replicated storage volume.

For example, a given hash digest can be generated by application of a hash function such as the well-known Secure Hashing Algorithm 1 (SHA1) to the content of its corresponding data page. Other types of secure hashing algorithms, such as SHA2 or SHA256, or more generally other hash functions, can be used in generating content-based signatures herein.

A given hash digest in illustrative embodiments is unique to the particular content of the page from which it is generated, such that two pages with exactly the same content will have the same hash digest, while two pages with different content will have different hash digests. It is also possible that other types of content-based signatures may be used, such as hash handles of the type described elsewhere herein. A hash handle generally provides a shortened representation of its corresponding hash digest. More particularly, the hash handles are shorter in length than respective hash digests that are generated by applying a secure hashing algorithm to respective ones of the data pages. Hash handles are considered examples of “content-based signatures” as that term is broadly used herein.

The content-based signatures utilized in the bidirectional content-based signature comparisons illustratively comprise respective hash handles of respective ones of the data pages. In such embodiments, content-based signatures of a first type are utilized in detecting failures in data verification operations, and content-based signatures of a second type different than the first type are utilized in performing the bidirectional comparisons, with full hash digests being utilized in the former failure detection and lightweight hash handles being utilized in the latter bidirectional comparisons, although other arrangements are possible.

For example, in other embodiments, the same type of content-based signatures, such as full hash digests or lightweight hash handles, can be used for both failure detection and bidirectional comparisons.

In some embodiments, it is assumed that the source storage system 102S is a first storage system associated with a first site selected as a “winning site” in a replication failure recovery process and the target storage system 102T comprises a second storage system associated with a second site selected as a “losing site” in the replication failure recovery process. It is also possible that the target storage system 102T could instead be selected as the winning site and the source storage system 102S could instead be selected as the losing site, in which case their respective sets of processing operations performed in conjunction with implementing stale data detection and clean-up functionality would be reversed. Thus, in some embodiments, each of the storage systems 102 is configured in substantially the same manner, so as to support selective implementation of either one of winning site or losing site portions of the stale data detection and clean-up functionality.

The above-noted replication failure recovery process is illustratively initiated responsive to detection of the failure in the data verification operation. A current replication cycle of the ongoing asynchronous or synchronous replication process carried out between the source and target storage systems 102 is assumed to be suspended in conjunction with initiation of the replication failure recovery process.

Examples of the bidirectional content-based signature comparisons performed between data pages of the source and target storage systems 102 in illustrative embodiments will now be described in further detail.

As a first part of the bidirectional content-based signature comparisons performed in these examples, hash handles and logical addresses for respective data pages of the source storage system 102S are sent to the target storage system 102T, and a determination is made in the target storage system 102T as to whether or not data pages having the hash handles exist at the corresponding logical addresses in the target storage system 102T.

Any of the data pages for which it is determined that their respective hash handles do not exist at the corresponding logical addresses in the target storage system 102T are sent from the source storage system 102S to the target storage system 102T.

As a second part of the bidirectional content-based signature comparisons performed in these example, hash handles and logical addresses for respective data pages of the target storage system 102T are sent to the source storage system 102S, and a determination is made in the source storage system 102S as to whether or not data pages having the hash handles exist at the corresponding logical addresses in the source storage system 102S.

Any of the data pages for which it is determined that their respective hash handles do not exist at the corresponding logical addresses in the source storage system 102S are designated as stale data in the target storage system 102T.

These examples assume that the source storage system 102S is the winning site and the target storage system 102T is the losing site, although as indicated previously those roles can be reversed in other embodiments.

As indicated above, some embodiments utilize address metadata to provide content addressable storage functionality. The address metadata in some embodiments comprises at least a portion of one or more logical layer mapping tables that map logical addresses of respective ones of the data pages of the storage volume to corresponding content-based signatures of the respective data pages. Examples of logical layer mapping tables and other metadata structures maintained by at least the storage controller 108T of target storage system 102T will be described below in conjunction with FIGS. 3A through 3D.

The stale data detection and clean-up functionality implemented in system 100 is configured to operate in conjunction with an ongoing asynchronous or synchronous replication process without unduly lengthening any replication cycle or undermining the ability to meet a desired RPO. Moreover, such arrangements avoid wasting scarce network bandwidth resources in transferring over the network 104 any data pages that are already resident in the target storage system 102T.

A more particular example of the stale data detection and clean-up functionality described above will now be presented. In this example, the replication control logic instances 112S and 112T are assumed to cooperate to implement the stale data detection and clean-up functionality, and collectively provide a replication engine of system 100 that can replicate one or more storage volumes from one of the storage systems 102 to the other one of the storage systems, and vice-versa. Accordingly, the designation of one of the storage systems 102 as the “source” and the other as the “target” can vary over time. For example, a particular one of the storage systems 102 designated as a “winning” site in a given replication failure recovery scenario can become the source storage system even though it was previously designated as the target storage system prior to the failure. Similarly, a particular one of the storage systems 102 designated as a “losing” site in a replication failure recovery scenario can become the target storage system even though it was previously designated as the source storage system prior to the failure.

In some replication failure recovery scenarios, the storage systems 102 can become disconnected from one another due to a communication failure, while writes continue to be made to one of the storage systems 102 from one or more of the host devices 101, such that one of the sites will contain data not present in the other site. Numerous other types of failures can result in differences between the data pages of one or more replicated storage volumes of the two systems. After detection of such a failure, it is generally necessary to bring the two systems back into synchronization such that the replication process can resume, and this illustratively involves picking one of the systems as the “winner” with the other being the “loser.” The winner then operates as a source storage system and replicates its data to the loser which operates as a target storage system, and the target storage system needs to clean up its stale data that is not present at the source storage system.

The stale data detection and clean-up functionality disclosed herein considerably facilitates this transitional operation of the source and target storage systems in recovering from the detected failure. For example, the illustrative embodiments described above provide bandwidth-efficient techniques for detecting and cleaning up stale data via bidirectional hash-based data verification. Such arrangements are particularly effective, even in cases of sparsely-allocated storage volumes. As indicated previously, the disclosed techniques are applicable to asynchronous and synchronous replication, including active-active replication.

The present example of stale data detection and clean-up functionality in an illustrative embodiment comprises an algorithm that includes the following steps, which are performed responsive to the detection of a failure in a data verification operation:

1. Suspend replication cycle.

2. Perform hash-based comparison of data pages from source to target, with source sending hash handles and offset information for all allocated data pages of one or more replicated storage volumes on source to target.

3. For any data page for which the target does not have the data page allocated or finds a hash handle mismatch for the data page, the source sends that data page to target.

4. Perform hash-based comparison of data pages from target to source, with source sending hash handles and offset information for all allocated data pages of one or more replicated storage volumes on target to source.

5. For any data page for which the source does not have the data page allocated or finds a hash handle mismatch for the data page, the target designates that data page as stale data and initiates a clean-up operation for the stale data in the manner described elsewhere herein.

The failure in the data verification operation that triggers the above example algorithm in illustrative embodiments comprises detection of a hash digest mismatch between one or more data pages on the source and one or more corresponding data pages on the target. Other failure detection techniques can be used in other embodiments. Also, although reference is made to one or more replicated storage volumes, this can include in some embodiments all of the data of the source and target storage systems.

The example algorithm provided above is particularly efficient in those situations in which the one or more replicated storage volumes are sparsely populated, with large chunks of unallocated logical address space. More particularly, in the bidirectional hash-based comparisons of Steps 2 and 4 of the example algorithm above, such large chunks of unallocated logical address space can simply be skipped over, thereby achieving highly efficient iterated scanning of the one or more replicated storage volumes.

Such an arrangement advantageously avoids the need for inefficient operations, such as a byte-to-byte comparison of source and target data, which is both time and bandwidth consuming and may not always feasible as it requires a byte comparison application to have access to both source and target data. These and other approaches also fail to provide efficient detection and clean-up of stale data on the target that does not exist on the source. It is to be appreciated that the particular algorithm steps above are examples only, and other arrangements of steps or process operations can be used in other embodiments.

The above-described operations carried out in conjunction with a process for stale data detection and clean-up involving the storage systems 102 are illustratively performed at least in part under the control of the replication engine comprising the multiple instances of replication control logic 112. As is apparent from the foregoing description, such operations utilize hash digests or other types of content-based signatures generated by one or more of the signature generators 114.

The storage systems 102 in the FIG. 1 embodiment are assumed to be implemented using at least one processing platform each comprising one or more processing devices each having a processor coupled to a memory. Such processing devices can illustratively include particular arrangements of compute, storage and network resources.

The storage systems 102 may be implemented on respective distinct processing platforms, although numerous other arrangements are possible. At least portions of their associated host devices may be implemented on the same processing platforms as the storage systems 102 or on separate processing platforms.

The term “processing platform” as used herein is intended to be broadly construed so as to encompass, by way of illustration and without limitation, multiple sets of processing devices and associated storage systems that are configured to communicate over one or more networks. For example, distributed implementations of the system 100 are possible, in which certain components of the system reside in one data center in a first geographic location while other components of the system reside in one or more other data centers in one or more other geographic locations that are potentially remote from the first geographic location. Thus, it is possible in some implementations of the system 100 for the storage systems 102 to reside in different data centers. Numerous other distributed implementations of the storage systems 102 and their respective associated sets of host devices are possible.

Additional examples of processing platforms utilized to implement storage systems and possibly their associated host devices in illustrative embodiments will be described in more detail below in conjunction with FIGS. 5 and 6.

It is to be appreciated that these and other features of illustrative embodiments are presented by way of example only, and should not be construed as limiting in any way.

Accordingly, different numbers, types and arrangements of system components such as host devices 101, storage systems 102, network 104, storage devices 106, storage controllers 108 and storage volumes 110 can be used in other embodiments.

It should be understood that the particular sets of modules and other components implemented in the system 100 as illustrated in FIG. 1 are presented by way of example only. In other embodiments, only subsets of these components, or additional or alternative sets of components, may be used, and such components may exhibit alternative functionality and configurations.

For example, in other embodiments, functionality for stale data detection and clean-up can be implemented in one or more host devices, or partially in a host device and partially in a storage system. Accordingly, illustrative embodiments are not limited to arrangements in which all such functionality is implemented in source and target storage systems or a host device, and therefore encompass various hybrid arrangements in which the functionality is distributed over one or more storage systems and one or more associated host devices, each comprising one or more processing devices.

As another example, it is possible in some embodiments that the source storage system and the target storage system can comprise the same storage system. In such an arrangement, an asynchronous or synchronous replication process is illustratively implemented to replicate data from one portion of the storage system to another portion of the storage system. The terms “source storage system” and “target storage system” as used herein are therefore intended to be broadly construed so as to encompass such possibilities.

The operation of the information processing system 100 will now be described in further detail with reference to the flow diagram of the illustrative embodiment of FIG. 2, which implements a stale data detection and clean-up process in conjunction with ongoing asynchronous or synchronous replication. The steps of the process illustratively involve interactions between a source storage system and a target storage system, referred to as respective “source” and “target” in these figures, illustratively utilizing replication control logic instances and signature generators of storage controllers of the source and target. It is possible in other embodiments that at least one of the storage systems does not include replication control logic and a signature generator, and in such embodiments these components are instead implemented in one or more host devices. The source and target storage systems in this embodiment are assumed to comprise respective winning and losing sites in a replication failure recovery process, but as indicated elsewhere herein, those roles can be reversed in other embodiments.

The stale data detection and clean-up process as illustrated in FIG. 2 includes steps 200 through 214, and is suitable for use in system 100 but is more generally applicable to a wide variety of other types of information processing systems in which data is replicated between multiple storage systems.

In step 200, a failure in ongoing replication of at least one storage volume from source to target is detected and replication is suspended. The ongoing replication can comprise asynchronous or synchronous replication. The one or more storage volumes subject to ongoing replication are illustratively part of a consistency group that is subject to asynchronous or synchronous replication from the source to the target. For example, one or more sets of storage volumes each comprising a plurality of storage volumes can be included in a given consistency group. It will be assumed for clarity and simplicity of the following description that at least one storage volume is subject to ongoing replication, but the techniques described can be extended in a straightforward manner to multiple identified storage volumes as well as multiple sets of storage volumes.

In step 202, the winning site is designated as the source storage system (“source”) and the losing site is designated as target storage system (“target”). This selection of winning and losing sites is illustratively performed as part of a failure recovery process triggered responsive to the detected failure.

In step 204, the source sends hash handles and offsets for source data pages to the target. The offsets illustratively comprise at least portions of logical addresses of the source data pages within a given LUN or other logical storage volume. The hash handle for a given one of the source data pages at a particular offset represents that data page and is determined based at least in part on content of that data page as described elsewhere herein. Such a hash handle is an example of what is more generally referred to herein as a “content-based signature” of the corresponding data page.

In step 206, for any source data page for which the corresponding hash handle is not at the expected offset in the target, that data page is sent from the source to the target. Accordingly, the remaining data pages that are found to already exist at the expected offsets in the target, based on their respective corresponding hash handles as sent from the source, are not sent from the source to the target. Such an arrangement advantageously conserves network bandwidth between the source and target.

In step 208, the target sends hash handles and offsets for target data pages to the source. Again, the offsets illustratively comprise at least portions of logical addresses of the source data pages within a given LUN or other logical storage volume. The hash handle for a given one of the target data pages at a particular offset represents that data page and is determined based at least in part on content of that data page as described elsewhere herein.

In step 210, for any target data page for which the corresponding hash handle is not at expected offset in the source, that data page is designated as stale data on the target.

Steps 204 through 210 implement an illustrative example of what is more generally referred to herein as bidirectional content-based signature comparisons between data pages of a source storage system and data pages of a target storage system. Other types of bidirectional content-based signature comparisons can be used in other embodiments. For example, although the bidirectional comparisons in this example are based on hash handles of respective data pages, other embodiments can implement such comparisons using other types of content-based signatures, such as hash digests.

In step 212, clean-up of the designated stale data on the target is initiated. For example, the clean-up can be initiated on a per-page basis, such that each data page designated as stale data is immediately deleted, released or otherwise cleaned up. As another example, the clean-up can be initiated for groups of multiple data pages, possibly responsive to a designated threshold number of data pages being designated as stale data. Accordingly, a wide variety of different clean-up triggering arrangements can be used. Such arrangements in some embodiments can involve issuing a trim operation, an unmap operation or another type of operation directed to a given data page or set of data pages designated as stale data.

In step 214, replication from the source to the target is restarted.

Additional instances of the process of FIG. 2 can be performed responsive to detection of respective additional failures in the replication once it is restarted after suspension for a previous failure.

The particular processing operations and other system functionality described in conjunction with the flow diagram of FIG. 2 are presented by way of illustrative example only, and should not be construed as limiting the scope of the disclosure in any way. Alternative embodiments can use other types of processing operations for stale data detection and clean-up in conjunction with asynchronous or synchronous replication. For example, the ordering of the process steps may be varied in other embodiments, or certain steps may be performed at least in part concurrently with one another rather than serially. Also, one or more of the process steps may be repeated periodically, or multiple instances of the process can be performed in parallel with one another in order to implement a plurality of different asynchronous or synchronous replication processes for respective different sets of one or more storage volumes or for different storage systems or portions thereof within a given information processing system.

Functionality such as that described in conjunction with the flow diagram of FIG. 2 can be implemented at least in part in the form of one or more software programs stored in memory and executed by a processor of a processing device such as a computer or server. As will be described below, a memory or other storage device having executable program code of one or more software programs embodied therein is an example of what is more generally referred to herein as a “processor-readable storage medium.”

For example, storage controllers such as storage controllers 108 of storage systems 102 that are configured to control performance of one or more steps of the FIG. 2 process in their corresponding system 100 can be implemented as part of what is more generally referred to herein as a processing platform comprising one or more processing devices each comprising a processor coupled to a memory. A given such processing device may correspond to one or more virtual machines or other types of virtualization infrastructure such as Docker containers or Linux containers (LXCs). The storage controllers 108, as well as other system components, may be implemented at least in part using processing devices of such processing platforms. For example, in a distributed implementation of a given one of the storage controllers 108, respective distributed modules of such a storage controller can be implemented in respective containers running on respective ones of the processing devices of a processing platform.

The FIG. 2 process makes use of various metadata structures that are maintained within the source and target storage systems. Examples of metadata structures maintained by the source and target storage systems in illustrative embodiments include the logical layer and physical layer mapping tables shown in respective FIGS. 3A, 3B, 3C and 3D. It is to be appreciated that these particular tables are only examples, and other tables or metadata structures having different configurations of entries and fields can be used in other embodiments.

Referring initially to FIG. 3A, an address-to-hash (“A2H”) table 300 is shown. The A2H table 300 comprises a plurality of entries accessible utilizing logical addresses denoted Logical Address 1, Logical Address 2, . . . Logical Address M as respective keys, with each such entry of the A2H table 300 comprising a corresponding one of the logical addresses, a corresponding one of the hash handles, and possibly one or more additional fields.

FIG. 3B shows a hash-to-data (“H2D”) table 302 that illustratively comprises a plurality of entries accessible utilizing hash handles denoted Hash Handle 1, Hash Handle 2, . . . Hash Handle D as respective keys, with each such entry of the H2D table 302 comprising a corresponding one of the hash handles, a physical offset of a corresponding one of the data pages, and possibly one or more additional fields.

Referring now to FIG. 3C, a hash metadata (“HMD”) table 304 comprises a plurality of entries accessible utilizing hash handles denoted Hash Handle 1, Hash Handle 2, . . . Hash Handle H as respective keys. Each such entry of the HMD table 304 comprises a corresponding one of the hash handles, a corresponding reference count and a corresponding physical offset of one of the data pages. A given one of the reference counts denotes the number of logical pages in the storage system that have the same content as the corresponding data page and therefore point to that same data page via their common hash digest. Although not explicitly so indicated in the figure, the HMD table 304 may also include one or more additional fields.

In the present embodiment, the HMD table of FIG. 3C illustratively comprises at least a portion of the same information that is found in the H2D table of FIG. 3B. Accordingly, in other embodiments, those two tables can be combined into a single table, illustratively referred to as an H2D table, an HMD table or another type of physical layer mapping table providing a mapping between hash values, such as hash handles or hash digests, and corresponding physical addresses of data pages.

FIG. 3D shows a physical layer based (“PLB”) table 306 that illustratively comprises a plurality of entries accessible utilizing physical offsets denoted Physical Offset 1, Physical Offset 2, . . . Physical Offset P as respective keys, with each such entry of the PLB table 306 comprising a corresponding one of the physical offsets, a corresponding one of the hash digests, and possibly one or more additional fields.

As indicated above, the hash handles are generally shorter in length than the corresponding hash digests of the respective data pages, and each illustratively provides a short representation of the corresponding full hash digest. For example, in some embodiments, the full hash digests are 20 bytes in length, and their respective corresponding hash handles are illustratively only 4 or 6 bytes in length.

Also, it is to be appreciated that terms such as “table” and “entry” as used herein are intended to be broadly construed, and the particular example table and entry arrangements of FIGS. 3A through 3D can be varied in other embodiments. For example, additional or alternative arrangements of entries can be used.

In some embodiments, the storage system comprises an XtremIO™ storage array or other type of content addressable storage system suitably modified to incorporate stale data detection and clean-up functionality as disclosed herein.

An illustrative embodiment of such a content addressable storage system will now be described with reference to FIG. 4. In this embodiment, a content addressable storage system 405 comprises a plurality of storage devices 406 and an associated storage controller 408. The content addressable storage system 405 may be viewed as a particular implementation of a given one of the storage systems 102, and accordingly is assumed to be coupled to the other one of the storage systems 102 and to one or more host devices of a computer system within information processing system 100.

Although it is assumed that both the source storage system 102S and the target storage system 102T are content addressable storage systems in some embodiments, other types of storage systems can be used for one or both of the source storage system 102S and the target storage system 102T in other embodiments. For example, it is possible that at least one of the storage systems 102 in an illustrative embodiment need not be a content addressable storage system and need not include an ability to generate content-based signatures. In such an embodiment, the signature generator and other parts of the stale data detection and clean-up functionality of the one or more storage systems can be implemented in a host device.

The storage controller 408 in the present embodiment is configured to implement stale data detection and clean-up functionality of the type previously described in conjunction with FIGS. 1 through 3. For example, the content addressable storage system 405 illustratively participates as a source storage system in an asynchronous or synchronous replication process with a target storage system that may be implemented as another instance of the content addressable storage system 405.

The storage controller 408 includes distributed modules 412 and 414, which are configured to operate in a manner similar to that described above for respective corresponding replication control logic 112 and signature generators 114 of the storage controllers 108 of system 100. Module 412 is more particularly referred to as distributed replication control logic, and illustratively comprises multiple replication control logic instances on respective ones of a plurality of distinct nodes, with the multiple replication control logic instances comprising at least a portion of a replication engine configured to perform process operations such as those described in conjunction with FIG. 2. Module 414 is more particularly referred to as a distributed signature generator, and illustratively comprises multiple signature generation instances on respective ones of the distinct nodes.

The content addressable storage system 405 in the FIG. 4 embodiment is implemented as at least a portion of a clustered storage system and includes a plurality of storage nodes 415 each comprising a corresponding subset of the storage devices 406. Such storage nodes 415 are examples of the “distinct nodes” referred to above, and other clustered storage system arrangements comprising multiple storage nodes and possibly additional or alternative nodes can be used in other embodiments. A given clustered storage system may therefore include not only storage nodes 415 but also additional storage nodes, compute nodes or other types of nodes coupled to network 104. Alternatively, such additional storage nodes may be part of another clustered storage system of the system 100. Each of the storage nodes 415 of the storage system 405 is assumed to be implemented using at least one processing device comprising a processor coupled to a memory. he storage controller 408 of the content addressable storage system 405 is implemented in a distributed manner so as to comprise a plurality of distributed storage controller components implemented on respective ones of the storage nodes 415. The storage controller 408 is therefore an example of what is more generally referred to herein as a “distributed storage controller.” In subsequent description herein, the storage controller 408 is referred to as distributed storage controller 408.

Each of the storage nodes 415 in this embodiment further comprises a set of processing modules configured to communicate over one or more networks with corresponding sets of processing modules on other ones of the storage nodes 415. The sets of processing modules of the storage nodes 415 collectively comprise at least a portion of the distributed storage controller 408 of the content addressable storage system 405.

The modules of the distributed storage controller 408 in the present embodiment more particularly comprise different sets of processing modules implemented on each of the storage nodes 415. The set of processing modules of each of the storage nodes 415 comprises at least a control module 408C, a data module 408D and a routing module 408R. The distributed storage controller 408 further comprises one or more management (“MGMT”) modules 408M. For example, only a single one of the storage nodes 415 may include a management module 408M. It is also possible that management modules 408M may be implemented on each of at least a subset of the storage nodes 415. A given set of processing modules implemented on a particular one of the storage nodes 415 therefore illustratively includes at least one control module 408C, at least one data module 408D and at least one routing module 408R, and possibly a management module 408M.

Communication links may be established between the various processing modules of the distributed storage controller 408 using well-known communication protocols such as IP, Transmission Control Protocol (TCP), and remote direct memory access (RDMA). For example, respective sets of IP links used in data transfer and corresponding messaging could be associated with respective different ones of the routing modules 408R.

Although shown as separate modules of the distributed storage controller 408, the modules 412 and 414 in the present embodiment are assumed to be distributed at least in part over at least a subset of the other modules 408C, 408D, 408R and 408M of the storage controller 408.

Accordingly, at least portions of the stale data detection and clean-up functionality of the modules 412 and 414 may be implemented in one or more of the other modules of the storage controller 408. In other embodiments, the modules 412 and 414 may be implemented as stand-alone modules of the storage controller 408.

The storage devices 406 are configured to store metadata pages 420 and user data pages 422, and may also store additional information not explicitly shown such as checkpoints and write journals. The metadata pages 420 and the user data pages 422 are illustratively stored in respective designated metadata and user data areas of the storage devices 406. Accordingly, metadata pages 420 and user data pages 422 may be viewed as corresponding to respective designated metadata and user data areas of the storage devices 406.

A given “page” as the term is broadly used herein should not be viewed as being limited to any particular range of fixed sizes. In some embodiments, a page size of 8 kilobytes (KB) is used, but this is by way of example only and can be varied in other embodiments. For example, page sizes of 4 KB, 16 KB or other values can be used. Accordingly, illustrative embodiments can utilize any of a wide variety of alternative paging arrangements for organizing the metadata pages 420 and the user data pages 422.

The user data pages 422 are part of a plurality of LUNs configured to store files, blocks, objects or other arrangements of data, each also generally referred to herein as a “data item,” on behalf of users of the content addressable storage system 405. Each such LUN may comprise particular ones of the above-noted pages of the user data area. The user data stored in the user data pages 422 can include any type of user data that may be utilized in the system 100. The term “user data” herein is therefore also intended to be broadly construed.

A given storage volume for which content-based signatures are generated using modules 412 and 414 illustratively comprises a set of one or more LUNs, each including multiple ones of the user data pages 422 stored in storage devices 406.

The content addressable storage system 405 in the embodiment of FIG. 4 is configured to generate hash metadata providing a mapping between content-based digests of respective ones of the user data pages 422 and corresponding physical locations of those pages in the user data area. Content-based digests generated using hash functions are also referred to herein as “hash digests.” Such hash digests or other types of content-based digests are examples of what are more generally referred to herein as “content-based signatures” of the respective user data pages 422. The hash metadata generated by the content addressable storage system 405 is illustratively stored as metadata pages 420 in the metadata area. The generation and storage of the hash metadata is assumed to be performed under the control of the storage controller 408.

Each of the metadata pages 420 characterizes a plurality of the user data pages 422. For example, a given set of user data pages representing a portion of the user data pages 422 illustratively comprises a plurality of user data pages denoted User Data Page 1, User Data Page 2, . . . User Data Page n. Each of the user data pages in this example is characterized by a LUN identifier, an offset and a content-based signature. The content-based signature is generated as a hash function of content of the corresponding user data page. Illustrative hash functions that may be used to generate the content-based signature include the above-noted SHA1 secure hashing algorithm, or other secure hashing algorithms known to those skilled in the art, including SHA2, SHA256 and many others. The content-based signature is utilized to determine the location of the corresponding user data page within the user data area of the storage devices 406.

Each of the metadata pages 420 in the present embodiment is assumed to have a signature that is not content-based. For example, the metadata page signatures may be generated using hash functions or other signature generation algorithms that do not utilize content of the metadata pages as input to the signature generation algorithm. Also, each of the metadata pages is assumed to characterize a different set of the user data pages.

A given set of metadata pages representing a portion of the metadata pages 420 in an illustrative embodiment comprises metadata pages denoted Metadata Page 1, Metadata Page 2, . . . , Metadata Page m, having respective signatures denoted Signature 1, Signature 2, . . . Signature m. Each such metadata page characterizes a different set of n user data pages. For example, the characterizing information in each metadata page can include the LUN identifiers, offsets and content-based signatures for each of the n user data pages that are characterized by that metadata page. It is to be appreciated, however, that the user data and metadata page configurations described above are examples only, and numerous alternative user data and metadata page configurations can be used in other embodiments.

Ownership of a user data logical address space within the content addressable storage system 405 is illustratively distributed among the control modules 408C.

The stale data detection and clean-up functionality provided by modules 412 and 414 in this embodiment is assumed to be distributed across multiple distributed processing modules, including at least a subset of the processing modules 408C, 408D, 408R and 408M of the distributed storage controller 408.

For example, the management module 408M of the storage controller 408 may include a replication control logic instance that engages corresponding replication control logic instances in all of the control modules 408C and routing modules 408R in order to implement an asynchronous or synchronous replication process.

In some embodiments, the content addressable storage system 405 comprises an XtremIO™ storage array suitably modified to incorporate stale data detection and clean-up functionality as disclosed herein.

In arrangements of this type, the control modules 408C, data modules 408D and routing modules 408R of the distributed storage controller 408 illustratively comprise respective C-modules, D-modules and R-modules of the XtremIO™ storage array. The one or more management modules 408M of the distributed storage controller 408 in such arrangements illustratively comprise a system-wide management module (“SYM module”) of the XtremIO™ storage array, although other types and arrangements of system-wide management modules can be used in other embodiments. Accordingly, stale data detection and clean-up functionality in some embodiments is implemented under the control of at least one system-wide management module of the distributed storage controller 408, utilizing the C-modules, D-modules and R-modules of the XtremIO™ storage array.

In the above-described XtremIO™ storage array example, each user data page has a fixed size such as 8 KB and its content-based signature is a 20-byte signature generated using the SHA1 secure hashing algorithm. Also, each page has a LUN identifier and an offset, and so is characterized by <lun_id, offset, signature>.

The content-based signature in the present example comprises a content-based digest of the corresponding data page. Such a content-based digest is more particularly referred to as a “hash digest” of the corresponding data page, as the content-based signature is illustratively generated by applying a hash function such as the SHA1 secure hashing algorithm to the content of that data page. The full hash digest of a given data page is given by the above-noted 20-byte signature. The hash digest may be represented by a corresponding “hash handle,” which in some cases may comprise a particular portion of the hash digest. The hash handle illustratively maps on a one-to-one basis to the corresponding full hash digest within a designated cluster boundary or other specified storage resource boundary of a given storage system. In arrangements of this type, the hash handle provides a lightweight mechanism for uniquely identifying the corresponding full hash digest and its associated data page within the specified storage resource boundary. The hash digest and hash handle are both considered examples of “content-based signatures” as that term is broadly used herein.

Examples of techniques for generating and processing hash handles for respective hash digests of respective data pages are disclosed in U.S. Pat. No. 9,208,162, entitled “Generating a Short Hash Handle,” and U.S. Pat. No. 9,286,003, entitled “Method and Apparatus for Creating a Short Hash Handle Highly Correlated with a Globally-Unique Hash Signature,” both of which are incorporated by reference herein.

As mentioned previously, storage controller components in an XtremIO™ storage array illustratively include C-module, D-module and R-module components. For example, separate instances of such components can be associated with each of a plurality of storage nodes in a clustered storage system implementation.

The distributed storage controller in this example is configured to group consecutive pages into page groups, to arrange the page groups into slices, and to assign the slices to different ones of the C-modules. For example, if there are 1024 slices distributed evenly across the C-modules, and there are a total of 16 C-modules in a given implementation, each of the C-modules “owns” 1024/16=64 slices. In such arrangements, different ones of the slices are assigned to different ones of the control modules 408C such that control of the slices within the storage controller 408 of the storage system 405 is substantially evenly distributed over the control modules 408C of the storage controller 408.

The D-module allows a user to locate a given user data page based on its signature. Each metadata page also has a size of 8KB and includes multiple instances of the <lun_id, offset, signature> for respective ones of a plurality of the user data pages. Such metadata pages are illustratively generated by the C-module but are accessed using the D-module based on a metadata page signature.

The metadata page signature in this embodiment is a 20-byte signature but is not based on the content of the metadata page. Instead, the metadata page signature is generated based on an 8-byte metadata page identifier that is a function of the LUN identifier and offset information of that metadata page.

If a user wants to read a user data page having a particular LUN identifier and offset, the corresponding metadata page identifier is first determined, then the metadata page signature is computed for the identified metadata page, and then the metadata page is read using the computed signature. In this embodiment, the metadata page signature is more particularly computed using a signature generation algorithm that generates the signature to include a hash of the 8-byte metadata page identifier, one or more ASCII codes for particular predetermined characters, as well as possible additional fields. The last bit of the metadata page signature may always be set to a particular logic value so as to distinguish it from the user data page signature in which the last bit may always be set to the opposite logic value.

The metadata page signature is used to retrieve the metadata page via the D-module. This metadata page will include the <lun_id, offset, signature> for the user data page if the user page exists. The signature of the user data page is then used to retrieve that user data page, also via the D-module.

Write requests processed in the content addressable storage system 405 each illustratively comprise one or more IO operations directing that at least one data item of the storage system 405 be written to in a particular manner. A given write request is illustratively received in the storage system 405 from a host device over a network. In some embodiments, a write request is received in the distributed storage controller 408 of the storage system 405, and directed from one processing module to another processing module of the distributed storage controller 408. For example, a received write request may be directed from a routing module 408R of the distributed storage controller 408 to a particular control module 408C of the distributed storage controller 408. Other arrangements for receiving and processing write requests from one or more host devices can be used.

The term “write request” as used herein is intended to be broadly construed, so as to encompass one or more IO operations directing that at least one data item of a storage system be written to in a particular manner. A given write request is illustratively received in a storage system from a host device.

In the XtremIO™ context, the C-modules, D-modules and R-modules of the storage nodes 415 communicate with one another over a high-speed internal network such as an InfiniBand network. The C-modules, D-modules and R-modules coordinate with one another to accomplish various IO processing tasks.

The write requests from the host devices identify particular data pages to be written in the storage system 405 by their corresponding logical addresses each comprising a LUN ID and an offset.

As noted above, a given one of the content-based signatures illustratively comprises a hash digest of the corresponding data page, with the hash digest being generated by applying a hash function to the content of that data page. The hash digest may be uniquely represented within a given storage resource boundary by a corresponding hash handle.

The content addressable storage system 405 utilizes a two-level mapping process to map logical block addresses to physical block addresses. The first level of mapping uses an address-to-hash (“A2H”) table and the second level of mapping uses a hash metadata (“HMD”) table, with the A2H and HMD tables corresponding to respective logical and physical layers of the content-based signature mapping within the content addressable storage system 405. The HMD table or a given portion thereof in some embodiments disclosed herein is more particularly referred to as a hash-to-data (“H2D”) table.

The first level of mapping using the A2H table associates logical addresses of respective data pages with respective content-based signatures of those data pages. This is also referred to as logical layer mapping.

The second level of mapping using the HMD table associates respective ones of the content-based signatures with respective physical storage locations in one or more of the storage devices 106. This is also referred to as physical layer mapping.

Examples of these and other metadata structures utilized in illustrative embodiments were described above in conjunction with FIGS. 3A through 3D. These particular examples include respective A2H, H2D, HMD and PLB tables. In some embodiments, the A2H and H2D tables are utilized primarily by the control modules 408C, while the HMD and PLB tables are utilized primarily by the data modules 408D.

For a given write request, hash metadata comprising at least a subset of the above-noted tables is updated in conjunction with the processing of that write request.

The A2H, H2D, HMD and PLB tables described above are examples of what are more generally referred to herein as “mapping tables” of respective distinct types. Other types and arrangements of mapping tables or other content-based signature mapping information may be used in other embodiments.

Such mapping tables are still more generally referred to herein as “metadata structures” of the content addressable storage system 405. It should be noted that additional or alternative metadata structures can be used in other embodiments. References herein to particular tables of particular types, such as A2H, H2D, HMD and PLB tables, and their respective configurations, should be considered non-limiting and are presented by way of illustrative example only. Such metadata structures can be implemented in numerous alternative configurations with different arrangements of fields and entries in other embodiments.

The logical block addresses or LBAs of a logical layer of the storage system 405 correspond to respective physical blocks of a physical layer of the storage system 405. The user data pages of the logical layer are organized by LBA and have reference via respective content-based signatures to particular physical blocks of the physical layer.

Each of the physical blocks has an associated reference count that is maintained within the storage system 405. The reference count for a given physical block indicates the number of logical blocks that point to that same physical block.

In releasing logical address space in the storage system, a dereferencing operation is generally executed for each of the LBAs being released. More particularly, the reference count of the corresponding physical block is decremented. A reference count of zero indicates that there are no longer any logical blocks that reference the corresponding physical block, and so that physical block can be released.

It should also be understood that the particular arrangement of storage controller processing modules 408C, 408D, 408R and 408M as shown in the FIG. 4 embodiment is presented by way of example only. Numerous alternative arrangements of processing modules of a distributed storage controller may be used to implement stale data detection and clean-up functionality in a clustered storage system in other embodiments.

Additional examples of content addressable storage functionality implemented in some embodiments by control modules 408C, data modules 408D, routing modules 408R and management module(s) 408M of distributed storage controller 408 can be found in U.S. Pat. No. 9,104,326, entitled “Scalable Block Data Storage Using Content Addressing,” which is incorporated by reference herein. Alternative arrangements of these and other storage node processing modules of a distributed storage controller in a content addressable storage system can be used in other embodiments.

Illustrative embodiments of a storage system with stale data detection and clean-up functionality as disclosed herein can provide a number of significant advantages relative to conventional arrangements.

For example, some of these embodiments advantageously provide increased efficiency in recovery from replication failures. Such embodiments perform replication failure recovery in a manner that ensures that any stale data is effectively eliminated or otherwise cleaned up as part of the replication failure recovery, even in cases involving sparsely-allocated storage. Moreover, such stale data detection and clean-up functionality is implemented in some embodiments in a manner that conserves potentially scarce network bandwidth resources between source and target storage systems.

These and other embodiments provide bandwidth efficient stale data detection and clean-up in conjunction with asynchronous or synchronous replication of one or more storage volumes from a source storage system to a target storage system. More particularly, illustrative embodiments disclosed herein can considerably reduce the amount of network bandwidth that is consumed in recovering from a replication failure, thereby leading to improved storage system performance.

In some embodiments, the source and target storage systems are illustratively implemented as respective content addressable storage systems, but in other embodiments one or more of the storage systems can instead be a traditional storage array, which does not support any type of content addressable storage functionality, and possibly does not even include any ability to generate hash digests, hash handles or other content-based signatures. For example, in embodiments in which at least one of the source storage system and the target storage system is unable to generate hash digests, hash handles or other content-based signatures, that functionality can be provided by a host device.

Accordingly, functionality for stale data detection and clean-up as disclosed herein can be implemented in at least one storage system, in at least one host device, or partially in one or more storage systems and partially in one or more host devices.

It is to be appreciated that the particular advantages described above and elsewhere herein are associated with particular illustrative embodiments and need not be present in other embodiments. Also, the particular types of information processing system features and functionality as illustrated in the drawings and described above are exemplary only, and numerous other arrangements may be used in other embodiments.

Illustrative embodiments of processing platforms utilized to implement host devices and storage systems with stale data detection and clean-up functionality will now be described in greater detail with reference to FIGS. 5 and 6. Although described in the context of system 100, these platforms may also be used to implement at least portions of other information processing systems in other embodiments.

FIG. 5 shows an example processing platform comprising cloud infrastructure 500. The cloud infrastructure 500 comprises a combination of physical and virtual processing resources that may be utilized to implement at least a portion of the information processing system 100. The cloud infrastructure 500 comprises multiple virtual machines (VMs) and/or container sets 502-1, 502-2, . . . 502-L implemented using virtualization infrastructure 504. The virtualization infrastructure 504 runs on physical infrastructure 505, and illustratively comprises one or more hypervisors and/or operating system level virtualization infrastructure. The operating system level virtualization infrastructure illustratively comprises kernel control groups of a Linux operating system or other type of operating system.

The cloud infrastructure 500 further comprises sets of applications 510-1, 510-2, . . . 510-L running on respective ones of the VMs/container sets 502-1, 502-2, . . . 502-L under the control of the virtualization infrastructure 504. The VMs/container sets 502 may comprise respective VMs, respective sets of one or more containers, or respective sets of one or more containers running in VMs.

In some implementations of the FIG. 5 embodiment, the VMs/container sets 502 comprise respective VMs implemented using virtualization infrastructure 504 that comprises at least one hypervisor. Such implementations can provide stale data detection and clean-up functionality of the type described above for one or more processes running on a given one of the VMs. For example, each of the VMs can implement replication control logic and/or signature generators for supporting stale data detection and clean-up functionality in the system 100.

An example of a hypervisor platform that may be used to implement a hypervisor within the virtualization infrastructure 504 is the VMware® vSphere® which may have an associated virtual infrastructure management system such as the VMware® vCenter™. The underlying physical machines may comprise one or more distributed processing platforms that include one or more storage systems.

In other implementations of the FIG. 5 embodiment, the VMs/container sets 502 comprise respective containers implemented using virtualization infrastructure 504 that provides operating system level virtualization functionality, such as support for Docker containers running on bare metal hosts, or Docker containers running on VMs. The containers are illustratively implemented using respective kernel control groups of the operating system. Such implementations can also provide stale data detection and clean-up functionality of the type described above. For example, a container host device supporting multiple containers of one or more container sets can implement one or more instances of replication control logic and/or signature generators for supporting stale data detection and clean-up functionality in the system 100.

As is apparent from the above, one or more of the processing modules or other components of system 100 may each run on a computer, server, storage device or other processing platform element. A given such element may be viewed as an example of what is more generally referred to herein as a “processing device.” The cloud infrastructure 500 shown in FIG. 5 may represent at least a portion of one processing platform. Another example of such a processing platform is processing platform 600 shown in FIG. 6.

The processing platform 600 in this embodiment comprises a portion of system 100 and includes a plurality of processing devices, denoted 602-1, 602-2, 602-3, . . . 602-K, which communicate with one another over a network 604.

The network 604 may comprise any type of network, including by way of example a global computer network such as the Internet, a WAN, a LAN, a satellite network, a telephone or cable network, a cellular network, a wireless network such as a WiFi or WiMAX network, or various portions or combinations of these and other types of networks.

The processing device 602-1 in the processing platform 600 comprises a processor 610 coupled to a memory 612.

The processor 610 may comprise a microprocessor, a microcontroller, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), graphics processing unit (GPU) or other type of processing circuitry, as well as portions or combinations of such circuitry elements.

The memory 612 may comprise random access memory (RAM), read-only memory (ROM), flash memory or other types of memory, in any combination. The memory 612 and other memories disclosed herein should be viewed as illustrative examples of what are more generally referred to as “processor-readable storage media” storing executable program code of one or more software programs.

Articles of manufacture comprising such processor-readable storage media are considered illustrative embodiments. A given such article of manufacture may comprise, for example, a storage array, a storage disk or an integrated circuit containing RAM, ROM, flash memory or other electronic memory, or any of a wide variety of other types of computer program products. The term “article of manufacture” as used herein should be understood to exclude transitory, propagating signals. Numerous other types of computer program products comprising processor-readable storage media can be used.

Also included in the processing device 602-1 is network interface circuitry 614, which is used to interface the processing device with the network 604 and other system components, and may comprise conventional transceivers.

The other processing devices 602 of the processing platform 600 are assumed to be configured in a manner similar to that shown for processing device 602-1 in the figure.

Again, the particular processing platform 600 shown in the figure is presented by way of example only, and system 100 may include additional or alternative processing platforms, as well as numerous distinct processing platforms in any combination, with each such platform comprising one or more computers, servers, storage devices or other processing devices.

For example, other processing platforms used to implement illustrative embodiments can comprise converged infrastructure such as VxRail™, VxRack™, VxRack™ FLEX, VxBlock™, or Vblock® converged infrastructure from Dell EMC.

It should therefore be understood that in other embodiments different arrangements of additional or alternative elements may be used. At least a subset of these elements may be collectively implemented on a common processing platform, or each such element may be implemented on a separate processing platform.

As indicated previously, components of an information processing system as disclosed herein can be implemented at least in part in the form of one or more software programs stored in memory and executed by a processor of a processing device. For example, at least portions of the stale data detection and clean-up functionality of one or more components of a storage system as disclosed herein are illustratively implemented in the form of software running on one or more processing devices.

It should again be emphasized that the above-described embodiments are presented for purposes of illustration only. Many variations and other alternative embodiments may be used. For example, the disclosed techniques are applicable to a wide variety of other types of information processing systems, host devices, storage systems, storage nodes, storage devices, storage controllers, asynchronous or synchronous replication processes, signature generators and associated control logic and metadata structures. Also, the particular configurations of system and device elements and associated processing operations illustratively shown in the drawings can be varied in other embodiments. Moreover, the various assumptions made above in the course of describing the illustrative embodiments should also be viewed as exemplary rather than as requirements or limitations of the disclosure. Numerous other alternative embodiments within the scope of the appended claims will be readily apparent to those skilled in the art. 

What is claimed is:
 1. An apparatus comprising: at least one processing device comprising a processor coupled to a memory; said at least one processing device being configured: to detect a failure in a data verification operation performed on at least a portion of at least one replicated storage volume; responsive to the detected failure, to perform bidirectional content-based signature comparisons between data pages of a source storage system and data pages of a target storage system; for one or more data pages of the source storage system for which respective content-based signatures are not found in the target storage system, to send the one or more data pages from the source storage system to the target storage system; and for one or more data pages of the target storage system for which respective content-based signatures are not found in the source storage system, to designate those data pages as stale data and to initiate a clean-up operation for the stale data.
 2. The apparatus of claim 1 wherein said at least one processing device is implemented at least in part within at least one of the source and target storage systems.
 3. The apparatus of claim 2 wherein said at least one processing device comprises a storage controller of at least one of the source storage system and the target storage system.
 4. The apparatus of claim 1 wherein the replicated storage volume comprises at least one logical storage volume that is part of a consistency group subject to an ongoing replication process carried out between the source and target storage systems.
 5. The apparatus of claim 1 wherein detection of the failure in the data verification operation comprises detection of a content-based signature mismatch between one or more data pages of the storage volume on the source storage system and one or more corresponding data pages of the storage volume on the target storage system.
 6. The apparatus of claim 5 wherein the content-based signatures utilized in the detection of the failure in the data verification operation comprise respective hash digests of respective data pages of the replicated storage volume with a given one of the hash digests being generated by applying a secure hashing algorithm to content of a corresponding one of the data pages of the replicated storage volume.
 7. The apparatus of claim 1 wherein the content-based signatures utilized in the bidirectional content-based signature comparisons comprise respective hash handles of respective ones of the data pages.
 8. The apparatus of claim 1 wherein the source storage system comprises a first storage system associated with a first site selected as a winning site in a replication failure recovery process and the target storage system comprises a second storage system associated with a second site selected as a losing site in the replication failure recovery process.
 9. The apparatus of claim 8 wherein the replication failure recovery process is initiated responsive to detection of the failure in the data verification operation.
 10. The apparatus of claim 8 wherein a current replication cycle of an ongoing replication process carried out between the source and target storage systems is suspended in conjunction with initiation of the replication failure recovery process.
 11. The apparatus of claim 1 wherein performing bidirectional content-based signature comparisons between data pages of the source and target storage systems comprises: sending hash handles and logical addresses for respective data pages of the source storage system to the target storage system; and determining if data pages having the hash handles exist at the corresponding logical addresses in the target storage system.
 12. The apparatus of claim 11 wherein any of the data pages for which it is determined that their respective hash handles do not exist at the corresponding logical addresses in the target storage system are sent from the source storage system to the target storage system.
 13. The apparatus of claim 1 wherein performing bidirectional content-based signature comparisons between data pages of the source and target storage systems comprises: sending hash handles and logical addresses for respective data pages of the target storage system to the source storage system; and determining if data pages having the hash handles exist at the corresponding logical addresses in the source storage system.
 14. The apparatus of claim 13 wherein any of the data pages for which it is determined that their respective hash handles do not exist at the corresponding logical addresses in the source storage system are designated as stale data in the target storage system.
 15. A method comprising: detecting a failure in a data verification operation performed on at least a portion of at least one replicated storage volume; responsive to the detected failure, performing bidirectional content-based signature comparisons between data pages of a source storage system and data pages of a target storage system; for one or more data pages of the source storage system for which respective content-based signatures are not found in the target storage system, sending the one or more data pages from the source storage system to the target storage system; and for one or more data pages of the target storage system for which respective content-based signatures are not found in the source storage system, designating those data pages as stale data and initiating a clean-up operation for the stale data; wherein the method is implemented by at least one processing device comprising a processor coupled to a memory.
 16. The method of claim 15 wherein performing bidirectional content-based signature comparisons between data pages of the source and target storage systems comprises: sending hash handles and logical addresses for respective data pages of the source storage system to the target storage system; and determining if data pages having the hash handles exist at the corresponding logical addresses in the target storage system.
 17. The method of claim 15 wherein performing bidirectional content-based signature comparisons between data pages of the source and target storage systems comprises: sending hash handles and logical addresses for respective data pages of the target storage system to the source storage system; and determining if data pages having the hash handles exist at the corresponding logical addresses in the source storage system.
 18. A computer program product comprising a non-transitory processor-readable storage medium having stored therein program code of one or more software programs, wherein the program code when executed by at least one processing device causes said at least one processing device: to detect a failure in a data verification operation performed on at least a portion of at least one replicated storage volume; responsive to the detected failure, to perform bidirectional content-based signature comparisons between data pages of a source storage system and data pages of a target storage system; for one or more data pages of the source storage system for which respective content-based signatures are not found in the target storage system, to send the one or more data pages from the source storage system to the target storage system; and for one or more data pages of the target storage system for which respective content-based signatures are not found in the source storage system, to designate those data pages as stale data and to initiate a clean-up operation for the stale data.
 19. The computer program product of claim 18 wherein performing bidirectional content-based signature comparisons between data pages of the source and target storage systems comprises: sending hash handles and logical addresses for respective data pages of the source storage system to the target storage system; and determining if data pages having the hash handles exist at the corresponding logical addresses in the target storage system.
 20. The computer program product of claim 18 wherein performing bidirectional content-based signature comparisons between data pages of the source and target storage systems comprises: sending hash handles and logical addresses for respective data pages of the target storage system to the source storage system; and determining if data pages having the hash handles exist at the corresponding logical addresses in the source storage system. 